The conversion of methanol to olefin-enriched or gasoline boiling range hydrocarbons using solid microporous molecular sieves is well known in the art. Such conversions using aluminosilicate, i.e., zeolitic, molecular sieve catalysts, are disclosed in U. S. Pat. Nos. 4,238,631; 4,328,384 and 4,423,274. The zeolite catalysts of these patented processes have pore diameters of about 6 Angstroms, a pore size believed to be too large for optimal olefin production. For that reason, a high coke level was maintained on the catalyst as a means of diminishing the effective pore diameters of the zeolites. In U.S. Pat. No. 4,079,095 the catalyst proposed for utilization in the conversion of methanol to light olefins is the relatively small pore zeolite ZSM-34. This particular zeolite did not benefit in olefin selectivity from being coked.
It has also been proposed to utilize certain of the aluminophosphate-based molecular sieves as catalysts for the conversion of alcohols to olefins. These microporous materials have crystal frameworks comprised of AlO.sub.2 and PO.sub.2 tetrahedral units and tetrahedral oxide units of other elements, most commonly silicon or silicon and one or more divalent or polyvalent metals. Of these so-called non-zeolitic molecular sieves, the most thoroughly investigated as catalysts for the methanol-to-olefins conversion reaction have been the silicoaluminophosphates or SAPO's. In U.S. Pat. No. 4,499,327 any known member of the SAPO-n subclass of aluminophosphates is proposed as being suitably employed, but a preference is stated for those SAPO species having pores large enough to adsorb xenon, but small enough to exclude isobutane. More preferred are those SAPO species in which the pores are large enough to exclude isobutane but admit n-hexane into the pore system. The species denominated SAPO-34 is among those of the most preferred group.
One of the most important embodiments of the methanol-to-olefins conversion process is directed to the production of light olefins, i.e., olefins containing from 2 to 4 carbon atoms, inclusive. Accordingly, it is important to utilize a catalyst which maximizes the production of these products, results in a high degree of conversion of the starting methanol, and does not deactivate rapidly under the process conditions imposed.